Radiant tube having uniform high-temperature distribution

ABSTRACT

Radiant tube heating apparatus in which rotation is imparted to a fuel-air mixture in a burner head and in which rotation continues after ignition as the flue gases travel in a helical path down the radiant tube. The continuing rotary action disrupts boundary layers permitting efficient heat transfer from flue gases to radiant tube, and further provides uniform temperature distribution along the tube. The rotation is initiated by helical channels formed in the peripheral surface of the burner head. Air is forced through those channels at high velocity and fuel is fed through the interior of the head and into the channels at right angles through a series of spaced orifices. The resultant complete mixing followed by near-adiabatic combustion create extremely high flame temperatures resulting in high radiant tube temperatures and high heat fluxes and heat release rates.

United States Patent Lazaros J. Lazaridis Lincoln, Mass.

Feb. 14, 1969 Mar. 16, 1971 Thermo Electron (Iorporation Waltharn, Mass.

Inventor Appl. No. Filed Patented Assignee RADIANT TUBE HAVING UNIFORM HIGH- TEMPERATURE DISTRIBUTION 5 Claims, 4 Drawing Figs.

US. Cl

Int. Cl F24c 3/00 Field ofSearch 126/91, 91

' References Cited UNITED STATES PATENTS 6/1964 Keough 3,195,609 7/1965 Nesbitt et al 431/9 3,267,927 8/1966 I-Iirschberg l26/91A FOREIGN PATENTS 846,237 8/1960 Great Britain 126/91A Primary Examiner-Charles J. Myhre AttorneyKenway, Jenney & Hildreth ABSTRACT: Radiant tube heating apparatus in which rotation is imparted to a fuel-air mixture in a burner head and in which rotation continues after ignition as the flue gases travel in a helical path down the radiant tube. The continuing rotary action disrupts boundary layers permitting efficient heat transfer from flue gases to radiant tube, and further provides uniform temperature distribution along the tube. The rotation is initiated by helical channels formed in the peripheral surface of the burner head. Air is forced through those channels at high velocity and fuel is fed through the interior of the head and into the channels at right angles through a series of spaced orifices. The resultant complete mixing followed by nearadiabatic combustion create extremely high flame tempera tures resulting in high radiant tube temperatures and high heat fluxes and heat release rates.

\ AIR 2 Patented March 16, 197 1 2 Sheets-Sheet l FIG. I

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INVENTOR. LAZAROS J. LAZARIDIS ATTORNEYS Patented March 16, 1971 3,570,471

2 Sheets-Sheet 2 w gig FIG. 4

INVENTOR.

LAZAROS J. LAZARIDIS ATTORNEYS RADIANT TUBE IIIAVDIG UNIFORM HIGH- TEMIPERATURE DISTRIBUTION In my copending application entitles Radiant Tube Furnace, Ser. No. 749,379, filed Aug. 1, 1968, I pointed out that electrical furnaces, and particularly induction furnaces, have frequently been preferred over conventional radiant tube furnaces in high temperature, (generally over 2,000 F.), applications. Several improvements designed to counter the trend toward electrical furnaces are recited in my c'opending application. These improvements are concerned principally with the maximization of heat transfer from the radiant tubes to the work being processed in the furnace. The present invention, although having the same general objectives of increasing the use and broadening the fields of use of radiant tube furnaces is more related to improving the radiant tube per se by raising its operating temperature and heat flux density well beyond what can be achieved with presently available combustion units of this type. The improved radiant tube apparatus is eminently suitable for use not only in the furnace disclosed in my cited application but also generally with radiant and immersiontype systems. g

Proper understanding of my invention requires consideration of the relationship of the radiant tube and burner. The radiant tube is cylindrical in shape as is the burner. At the peripheral surface of the burner, helical channels are formed. It is fitted snugly either directly into the radiant tube or into an enclosing sleeve which, with the channels forms a group of helical passages through which air or other combustion gas is forced. These passages serve the primary purpose of creating rotary motion of the fuel-mix which, after ignition, persists as a helical flue gas stream down the length of the radiant tube to enhance heat transfer and provide uniform temperature distribution along the radiant tube. Secondly, the passages in conjunction with fuel orifices normal to the channels aid in thorough mixing which increases efficiency and permit the achievement of high flame temperatures within the entire mass of the mixture.

The air or other combustion-supporting gas forced through the sleeve and formed by the channels into high velocity helical air streams is preferably delivered through a relatively large sleeve to the member enclosing the burner and the gas is delivered through a smaller sleeve communicating with the interior of the burner to reach the radial orifices.

The gas, emerging as it does, through the radial orifices at right angles to the turbulent airstream creates a vortex contributing to thorough mixing. The mixture as it leaves the burner head is rotating helically and is there ignited. Because of the thorough mixing, it burns almost adiabatically and products of combustion continue as flue gas streams down the radiant tube with the same generally helical motion imparted to the mixture. The rotation of the flue gas stream and the pressure against the interior wall of the radiant tube caused by the centrifugal force of the rotating stream scavenges away boundary layers to enhance heat transfer from the stream to the tube. Most important, transfer of heat from the stream to the tube is such that a uniform distribution of temperature along a tube of considerable length is achieved.

As a useful adjunct, a peep sight tube having a transparent seal at its end remote from the burner and terminating at the end wall of the burner head permits combustion to be observed and monitored. An electrode extending to a point external to that end wall to form a spark gap may be used for ignition. The electrode may pass through the burner head and out through the outer enclosing sleeve in, for example, a spark plug body to which conventional arrangements for electrical excitation may be made outside the enclosing sleeve.

My invention in its presently preferred form is described in detail below and the description should be read with reference to the accompanying drawing in which:

FIG. I is a view, partly in section, of a burner head suitable to be incorporated in my radiant tube burner system;

FIG. 2 is a fragmentary sectional view providing detail on the configuration of the burner head of FIG. 1;

FIG. 3 is an enlargement of the burner head of FIG. 1, and

FIG. 4 is a view, partly in section, illustrating the action of the flue gases in the radiant tube.

In FIG. 1, I show the introduction of air at the lower right through what may conveniently be a conventional pipe nipple 12. The pipe nipple may be, in turn, connected to a larger pipe or sleeve 14 through a wall of which a spark plug 16 is threaded. An electrode 13 of the spark plug is extended to pass parallel to the axis of the sleeve as is explained in greater detail below.

An opening is formed in the elbow of the pipe nipple 12 to accommodate a smaller pipe nipple 20 which may be welded or otherwise suitably sealed into the pipe nipple 12. A source of gas, not shown, is connected to the pipe nipple 20. The pipe nipple 20 also has an opening formed in its elbow to accommodate a tube 22 which may be of steel or other inexpensive metal. The tube 22, is also, of course, sealed into the opening in the nipple 20. A cap 24 is threaded upon the tube 22 and its end is sealed by an element of a quartz or other transparent material to permit observation of combustion through the tube 22.

1 A burner head 28 may be fitted axially into the radiant tube 22, but where it is desired to limit the length of the radiant tube, it may be fitted into a bridging ceramic adapter sleeve 26. The sleeve 26 is made of Mulfrax or other suitable hightemperature resistant material and has shoulders formed at its ends. The internal shoulder at the right end of the adapter 26 is fitted into the sleeve 14 and the external shoulder at the left end of the adapter 26 is fitted over a radiant tube 30. Onlya portion of the radiant tube 30 is visible, but it will be noted that the inside diameters of the adapter 26 and the radiant tube 30 are substantially the same and no discontinuity exists between the two.

The interconnection of the various elements with the burner head 28 is moreeasily seen in FIG. 2. As noted above, the

burner head 28 is snugly fitted into the adapter 26 and air coming through the adapter 26 at high velocity is forced into the desired rotation by the vanes 32 formed upon the outer surfaces of the burner head 28. The helical configuration of the vanes is made more obvious in FIG. 3.

Reverting to FIG. 2, it may be seen that the peep sight 22 is brazed or otherwise sealed in place in the end wall of the burner head 28 remote from the input side. The gasline 20, which may be formed with a suitable shoulder adjacent its end is fitted into an opening formed in the near wall of the burner head and it, too, is preferably brazed or welded in place. At a point off the axis but passing through both end walls of the burner head is a tube 36 within which is disposed a sleeve 38 of ceramic insulating material such as alundum. Within the ceramic sleeve the electrode extension 18 passes to a point beyond the far end wall where it is bent to form a spark gap. Radial openings 40 are formed through the cylindrical wall of the burner head 28 and, as is plain in FIG. 3. these orifices are spaced along the helical channels formed by the vanes 32.

In FIG. 4, I show in a somewhat idealized manner the effects obtained in my invention as a result of the thorough mixing, near-adiabatic combustion and, especially, the rotation of the products of combustion or flue gases, as they pass down the radiant tube 30. It will be noted that the burner head is recessed in its ceramic sleeve from the near end of the radiant tube. Also, as was noted above and is also plain in FIG. 4, the sleeve 26 surrounding and snugly fitted over the burner head 28 has an internal diameter closely matched to that of the radiant tube 30. Complete mixing and combustion are achieved before the'flue gasses reach the radiant tube and no discontinuities are present to interfere with the continued rotation of the flue gases. The larger the angle made by the helical channels with the axis of the cylindrical structure the shorter the length over which uniform temperature distribution will be had.

In point of fact, in a typical radiant tube of approximately 4 feet in length, the flue gas stream would go through approximately 10 turns where the burner head is formed with vanes at an angle of about6 0 to the axis of the head. It has been established experimehtally that with a tube of 4 feet in length,

3%inches inside diameter and 4 inches outside diameter, for example, the vane angle of 60 to the axis of the burner gave the highest temperature, flattest temperature profile and fastest heat up. Heat release densities of up to 3 million B.T.U. per hour per cubic foot of radiant tube volume have been achieved at a tube temperature of 3,050 F. and at a thennal efficiency of 15 percent measured calorimetrically. It should be also noted that by flue gas analysis complete combustion was verified, without flame extension beyond the end of the radiant tube. Uniform temperature distribution over widely varying lengths of radiant tube can be achieved by proper selection of the vane angle so that rotation of flue gas continues to the end of the radiant tube and boundary layer scavenging is more or less uniform over that entire length and not concentrated at the entry to the radiant tube.

Of course, the velocities of the air and the gas as well as other parameters including the number of orifices 40, and the diameters of the various components are interrelated in such a fashion that various changes and adjustments are feasible to optimum performance. By way of example with an experimental machined burner head, an air velocity of approximately 320 f.p.s. and a gas velocity at the injection orifices of approximately 185 f.p.s. have given excellent results. Insofar as the number of fuel injection orifices is concerned, a total of 24 such orifices each having a diameter of approximately 0.093 inches provided optimum performance. However, with changes of air velocity not only the size but the number of fuel injection orifices could be varied.

Although the various lines have been shown to be concentric, that is, the air input line, the gas input line, and the peep sight being constructed one within the other these need not be so arranged. It is desirable, however, to maintain the air input line as the outer enclosure, not only to form the desired helical passages with the burner head but also to provide a cooling effect. The cooling effect is so pronounced that the temperatures even at points just behind the burner head are several orders of magnitude lower than the temperatures at the radiant tube. Therefore, it is possible to use available, cheap materials such as conventional piping bringing the cost of the system to a point where the radiant tube burner unit and the furnace using it are competitive with other types of radiant tube combustors and furnaces. In some situations where cooling is of no great importance, preheated combustion air may be fed to the burner at temperatures approaching that of fuel ignition. In such circumstances, of course, it would be necessary to use materials capable of withstanding the high temperature of the combustion air. Also, of course, the helical channels may be formed in the sleeve surrounding the burner head rather than in the burner head per se, the important point being the helical openings at the interface for mixing and rotating the fuel mix. The design of the burner is such that all conventional safety equipment may be incorporated with a minimum of complication. Finally, although the foregoing description has been limited to radiant tube systems, it is obvious that the invention will find application in any immersion-type heaters of tubular configuration, especially where uniform temperature distribution and high heat flux density release to the tube is of importance.

Although I have disclosed my invention as embodied in specific structures, it should be limited not to those structures but rather only by the spirit and scope of the appended claims.

I claim:

1. In heating apparatus for a generally cylindrical radiant tube, a burner disposed axially of said tube for the combustion of a fuel mix, a sleeve closely surrounding said burner, helical channels being formed at the interface of said sleeve and said burner, means for forcing air through said helical channels and means for introducing fuel at right angles into said channels whereby products of combustion are formed into streams of flue gases proceeding helically through said radiant tube.

2. In heating apparatus for a generally cylindrical radiant tube, a burner disposed axially of said tube, a source of fuel, a

source of air, saidburner being hollow, and disposed adjacent an end of said radiant tube, said burner having a first plurality of helical channels formed on the outer surface thereof and a second plurality of radial openings formed therethrough, said openings being disposed at spaced points in said channels, means for forcing air from said source thereof through said helical channels and means for forcing gas from said source thereof through said radial openings to obtain a mixture of said air and gas in said channels and to impart rotation to said mixture, and means disposed adjacent said burner for igniting said mixture to form said rotating mixture into said stream of flue gases proceeding helically through said radiant tube.

3. In heating apparatus as defined in claim 2, the combination in which said burner is generally cylindrical in cross section and which further includes a sleeve having substantially the same internal diameter as said radiant tube surrounding said burner and aligned with said radiant tube.

4. In heating apparatus as defined in claim 2, the combination wherein said helical channels are at an angle of approximately 60 to the axis of said burner.

5. In heating apparatus as defined in claim 3, the combination which includes means connecting the interior of said burner head to said source of fuel and means connecting the interior of said sleeve to said source of air, the fiow of said air providing a cooling effect to said connecting means. 

1. In heating apparatus for a generally cylindrical radiant tube, a burner disposed axially of said tube for the combustion of a fuel mix, a sleeve closely surrounding said burner, helical channels being formed at the interface of said sleeve and said burner, means for forcing air through said helical channels and means for introducing fuel at right angles into said channels whereby products of combustion are formed into streams of flue gases proceeding helically through said radiant tube.
 2. In heating apparatus for a generally cylindrical radiant tube, a burner disposed axially of said tube, a source of fuel, a source of air, said burner being hollow, and disposed adjacent an end of said radiant tube, said burner having a first plurality of helical channels formed on the outer surface thereof and a second plurality of radial openings formed therethrough, said openings being disposed at spaced points in said channels, means for forcing air from said source thereof through said helical channels and means for forcing gas from said source thereof through said radial openings to obtain a mixture of said air and gas in said channels and to impart rotation to said mixture, and means disposed adjacent said burner for igniting said mixture to form said rotating mixture into said stream of flue gases proceeding helically through said radiant tube.
 3. In heating apparatus as defined in claim 2, the combination in which said burner is generally cylindrical in cross section and which further includes a sleeve having substantially the same internal diameter as said radiant tube surrounding said burner and aligned with said radiant tube.
 4. In heating apparatus as defined in claim 2, the combination wherein said helical channels are at an angle of approximately 60* to the axis of said burner.
 5. In heating apparatus as defined in claim 3, the combination which includes means connecting the interior of said burner head to said source of fuel and means connecting the interior of said sleeve to said source of air, the flow of said air providing a cooling effect to said connecting means. 